Navy Yard gunman Aaron Alexis originally attempted to purchase an assault rifle from a Virginia gun shop, the New York Times reported Tuesday, but had to settle for a pump-action shotgun because state law prohibited the sale of the military-style AR-15 to an out-of-state buyer, even though Alexis passed a background check.

Had Alexis instead gone online, he would have easily found about 200 such weapons for sale by private sellers, most within an hour’s drive of D.C., according to a TIME analysis of postings on ArmsList.com, the most popular site for prospective gun buyers and sellers to connect.

Though federal law typically limits private gun sales to in-state buyers, gun-control advocates like New York City Mayor Michael Bloomberg charge that the site enables illegal gun sales by giving ineligible buyers access to unscrupulous sellers. A December 2011 report by Bloomberg’s office found that 54 percent of ArmsList sellers were willing to sell to a person who admitted he probably would not pass a background check.

The Brady Center to Prevent Gun Violence recently sued the site—unsuccessfully—on behalf of a 36-year-old Illinois woman who was murdered by a man who obtained his gun illegally using the site.

Today, Mayors Against Illegal Guns released the results of an investigation of prospective buyers on Armslist.com. Instead of using private investigators posing as buyers, as the December 2011 report did, the group conducted its investigation by mining the phone numbers listed in wanted ads on the site. Researchers were able to positively connect 607 of those phone numbers to individuals. Background checks revealed that 1 in 30 of them had a criminal record that would have caused them to flunk the background check.

And wanted ads, New York City Mayor Michael Bloomberg noted in a press conference announcing the report, are only a small fraction of the site’s content.

“Think how many criminals are answering ‘want to sell ads,’ which they can do with complete anonymity,” he said.

To put the size of ArmsList.com in perspective, TIME downloaded all 98,537 posts that were active on the site on Tuesday, September 17. After removing all ads that appeared to be duplicates posted in different locations, the analysis found 15,729 rifles, 13,712 handguns, and 7,035 shotguns for sale in the United States just from private sellers. (About 5 percent of ads on ArmsList are posted by dealers). Here, you can see those figures broken out by state:

Methodology

ArmsList.com was mirrored between 10:00 AM EST and 6:00 PM EST on Tuesday, September 17. Posts were collated by unique seller using the site’s “Listings by this user” feature. To reduce double-counting of the same gun, which occurs most often when a person advertises the same firearm in multiple locations, only one gun per user of a given price was considered in the database. This means a person selling the same gun for two different prices in two different locations would still be counted twice, but that a person selling two guns for the same price would be undercounted. Both scenarios appear to be rare.